Support for Path MTU (PMTU) in the Path Computation
Element Communication Protocol (PCEP).Huawei TechnologiesDivyashree Techno Park, WhitefieldBangaloreKarnataka560066Indiadhruv.ietf@gmail.comHuawei TechnologiesHuawei Campus, No. 156 Beiqing Rd.Beijing100095Chinachengli13@huawei.comRouting
PCE Working GroupThe Path Computation Element (PCE) provides path computation
functions in support of traffic engineering in Multiprotocol Label
Switching (MPLS) and Generalized MPLS (GMPLS) networks.The Source Packet Routing in Networking (SPRING) architecture
describes how Segment Routing (SR) can be used to steer packets through
an IPv6 or MPLS network using the source routing paradigm. A Segment
Routed Path can be derived from a variety of mechanisms, including an
IGP Shortest Path Tree (SPT), explicit configuration, or a Path
Computation Element (PCE).Since the SR does not require signaling, the path maximum
transmission unit (MTU) information for SR path is not available. This
document specify the extension to PCE communication protocol (PCEP) to
carry path (MTU) in the PCEP messages.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP 14
when, and only when,
they appear in all capitals, as shown here. describes the Path Computation Element (PCE)
Communication Protocol (PCEP). PCEP enables the communication between a
Path Computation Client (PCC) and a PCE, or between PCE and PCE, for the
purpose of computation of Multiprotocol Label Switching (MPLS) as well
as Generalzied MPLS (GMPLS) Traffic Engineering Label Switched Path (TE
LSP) characteristics. specifies a set of extensions to PCEP to
enable stateful control of TE LSPs within and across PCEP sessions in
compliance with . It includes mechanisms to
effect LSP State Synchronization between PCCs and PCEs, delegation of
control over LSPs to PCEs, and PCE control of timing and sequence of
path computations within and across PCEP sessions. The model of
operation where LSPs are initiated from the PCE is described in .As per , with Segment
Routing (SR), a node steers a packet through an ordered list of
instructions, called segments. A segment can represent any instruction,
topological or service-based. A segment can have a semantic local to an
SR node or global within an SR domain. SR allows to enforce a flow
through any path and service chain while maintaining per-flow state only
at the ingress node of the SR domain. Segments can be derived from
different components: IGP, BGP, Services, Contexts, Locators, etc. The
SR architecture can be applied to the MPLS forwarding plane without any
change, in which case an SR path corresponds to an MPLS Label Switching
Path (LSP). The SR is applied to IPV6 forwarding plane using SRH. A SR
path can be derived from an IGP Shortest Path Tree (SPT), but SR-TE
paths may not follow IGP SPT. Such paths may be chosen by a suitable
network planning tool, or a PCE and provisioned on the ingress node.As per , it is possible
to use a stateful PCE for computing one or more SR-TE paths taking into
account various constraints and objective functions. Once a path is
chosen, the stateful PCE can initiate an SR-TE path on a PCC using PCEP
extensions specified in using the SR specific
PCEP extensions specified in . specifies PCEP extensions for
supporting a SR-TE LSP for MPLS data plane. extend PCEP to support SR
for IPv6 data plane.The maximum transmission unit (MTU) is the largest size packet or
frame, in bytes, that can be sent in a network. An MTU that is too large
might cause retransmissions. Too small an MTU might cause the router to
send and handle relatively more header overhead and acknowledgments.
When an LSP is created across a set of links with different MTU sizes,
the ingress router need to know what the smallest MTU is on the LSP
path. If this MTU is larger than the MTU of one of the intermediate
links, traffic might be dropped, because MPLS packets cannot be
fragmented. Also, the ingress router may not be aware of this type of
traffic loss, because the control plane for the LSP would still function
normally. specify the mechanism of MTU
signaling in RSVP.Since the SR does not require signaling, the path MTU information for
SR path is not available. This document specify the extension to PCEP to
carry path MTU in the PCEP messages. It is assumed that the PCE is aware
of the link MTU as part of the Traffic Engineering Database (TED)
population. This could be done via IGP, BGP-LS or some other means. Thus
the PCE can find the path MTU at the time of path computation and
include this information as part of the PCEP messages.Though the key use case for path MTU is SR, the PCEP extension (as
specified in this document) creates a new metric type for path MTU,
making this a generic extension that can be used independent of SR.The METRIC object is defined in Section 7.8 of , comprising metric-value and metric-type (T field),
and a flags field, comprising a number of bit flags (B bit and C bit).
This document defines a new type for the METRIC object for Path
MTU.T = TBD: Path MTU.A network comprises of a set of N links {Li, (i=1...N)}.A path P of a LSP is a list of K links {Lpi,(i=1...K)}.A Link MTU of link L is denoted M(L).A Path MTU metric for the path P = Min {M(Lpi), (i=1...K)}.The Path MTU metric type of the METRIC object in PCEP represents
the minimum of the Link MTU of all links along the path.When PCE computes the path, it can also find the Path MTU (based on
the above criteria) and include this information in the METRIC object
with the above metric type in the PCEP message when replying to the
PCC. In a Path Computation Reply (PCRep) message, the PCE MAY insert
the METRIC object with an Explicit Route Object (ERO) so as to provide
the METRIC (path MTU) for the computed path. The PCE MAY also insert
the METRIC object with a NO-PATH object to indicate that the metric
constraint could not be satisfied.Further, a PCC MAY use the Path MTU metric in a Path Computation
Request (PCReq) message to request a path meeting the MTU requirement
of the path. In this case, the B bit MUST be set to suggest a bound (a
maximum) for the Path MTU metric that must not be exceeded for the PCC
to consider the computed path as acceptable. The Path MTU metric must
be less than or equal to the value specified in the metric-value
field.A PCC can also use this metric to ask PCE to optimize the path MTU
during path computation. In this case, the B bit MUST be cleared.The error handling and processing of the METRIC object is as
specified in . specifies a set of extensions to PCEP to
enable stateful control of MPLS-TE and GMPLS LSPs via PCEP and the
maintaining of these LSPs at the stateful PCE. It further
distinguishes between an active and a passive stateful PCE. A passive
stateful PCE uses LSP state information learned from PCCs to optimize
path computations but does not actively update LSP state. In contrast,
an active stateful PCE utilizes the LSP delegation mechanism to update
LSP parameters in those PCCs that delegated control over their LSPs to
the PCE. describes the setup, maintenance,
and teardown of PCE-initiated LSPs under the stateful PCE model. The
document defines the PCInitiate message that is used by a PCE to
request a PCC to set up a new LSP.The new metric type defined in this document can also be used with
the stateful PCE extensions. The format of PCEP messages described in
and uses
<intended-attribute-list> and <attribute-list>,
respectively, (where the <intended-attribute-list> is the
attribute-list defined in Section 6.5 of .A PCE MAY include the path MTU metric in PCInitiate or PCUpd
message to inform the PCC of the path MTU calculated for the path. A
PCC MAY include the path MTU metric as a bound constraint or to
indicate optimization criteria (similar to PCReq).A Segment Routed path (SR path) can be derived from an IGP Shortest
Path Tree (SPT). Segment Routed Traffic Engineering paths (SR-TE
paths) may not follow IGP SPT. Such paths may be chosen by a suitable
network planning tool and provisioned on the source node of the SR-TE
path.It is possible to use a PCE for computing one or more SR-TE paths
taking into account various constraints and objective functions. Once
a path is chosen, the PCE can inform an SR-TE path on a PCC using PCEP
extensions specified in .
Further, adds the
support for IPv6 data plane in SR.The new metric type for path MTU is applicable for the SR-TE path
and require no additional extensions.This document defines a new METRIC type that do not add any new
security concerns beyond those discussed in in
itself. Some deployments may find the path MTU information to be extra
sensitive and could be used to influence path computation and setup with
adverse effect. Additionally, snooping of PCEP messages with such data
or using PCEP messages for network reconnaissance may give an attacker
sensitive information about the operations of the network. Thus, such
deployment should employ suitable PCEP security mechanisms like TCP
Authentication Option (TCP-AO) or Transport
Layer Security (TLS) . The procedure based on
TLS is considered a security enhancement and thus is much better suited
for the sensitive information.This document makes following requests to IANA for action.IANA maintains the "Path Computation Element Protocol (PCEP)
Numbers" registry. Within this registry, IANA maintains a subregistry
for "METRIC Object T Field". IANA is requested to make the following
allocation: